18/10/2024
GENERATOR SIZING GUIDE FOR OFF-GRID SOLAR SYSTEMS IN AUSTRALIA
Most off-grid solar systems are paired with a generator. Without utility power as a fallback, off-grid setups need a backup plan in case their solar system can’t produce enough to meet the property’s needs.
Petrol & Diesel generators are used in off-grid systems as a backup charging source to recharge the battery bank when solar can’t keep up with demand. They are also used for backup power in case equipment fails and the inverter system needs to be bypassed.
Here’s our advice on how to pick a generator for your off-grid solar system.
GENERATOR SIZING
As a general rule, the generator should be around 2 times the size of the inverter’s continuous output. For example, a 4,000-watt inverter should be paired with an 8,000-watt generator.
This is because the generator needs to charge batteries while still powering the loads (appliances using energy). If the loads total 4,000 watts, and the charger is 60 amps at 48 volts, that totals around 7kW of continuous power:
60a x 48v = 2,880 watts + 4,000 watts = 6,880 watts
8kW would make sense as a minimum generator size to power the loads and have enough power to charge the battery bank as well.
It’s a good idea to round up slightly to give some headroom for the generator.
CAN I USE A LARGER GENERATOR?
A larger generator can be beneficial if you have large loads like an air conditioner or a welder that will only be used when the generator is running.
We frequently sell 12kVA and 15kVA generators for an off-grid systems using a 4000w inverter. A larger generator is going to burn more fuel, but otherwise will work fine.
CAN I USE A SMALLER GENERATOR?
Smaller generators will still work, but it typically requires adjusting the settings to limit the battery charger’s output. Most battery chargers allow you to adjust the AC input amperage and charge rate.
You will need to match the generator’s voltage with the inverter. For example, 240Vac generators should be paired with 240Vac inverter/chargers.
In the rare case that you are using a three-phase generator or inverter, the same rules apply: the generator’s voltage would need to match the inverter.
Generator Fuel Type (Petrol, Diesel)
Most standby home generators work on Petrol or diesel. Diesel generators tend to be much more fuel efficient and longer lasting, but the initial cost can be 2-3 times more than a Petrol alternative.
The Powerlite 8kVA & 12kVA generators we sell to Off-grid customers typically use Petrol instead of diesel,
OTHER CONSIDERATIONS
WARRANTY
Most generators don’t have a warranty that covers off-grid or prime power applications. Check the generator warranty closely to be sure it can be used off the grid.
Powerlite’s PH080 Solar and PH120ET Solar generators are both warrantied for 24 months / 1000 hours of off-grid use (whichever comes first).
If you need more power, you can step up to the diesel enclosed units from 10kVA to 60kVA
2-WIRE START
Off-grid generators paired with solar power systems need 2-wire start capability to allow the automatic generator start (AGS) function to work. When your batteries drop below a certain voltage, the AGS kicks in to turn on the generator and recharge your battery bank.
Note that 2-wire start is different than electric start. Some generators will have a button for electric start/stop, but they can’t be controlled by a 2-wire signal, which means the inverter can’t communicate with the generator to trigger the AGS mechanism.
The Powerlite PH080 Solar and PH120ET Solar are both capable of two-wire start.
1500 RPM VS. 3000 RPM
Most generators operate at one of two engine speeds: 1500 or 3000 RPM. In Australia to produce 50Hz
The difference is based on engine design and the alternator being used. 1500 RPM generators are generally considered superior because they are more fuel-efficient, but they cost quite a bit more up front. 3000 RPM generators tend to be cheaper but less efficient.
The Powerlite PH080 Solar and PH120ET Solar generators we sell are 3000 RPM. They are very durable, reliable and more cost-effective for typical off-grid applications. Powerlite specialises in manufacturing off-grid 1500 RPM machines, although there may be options from other manufacturers. Be sure to do your research and read reviews to learn about the company and make sure you are getting a quality product.
CONCLUSION
When searching for a generator to support your off-grid solar system, keep a few things in mind:
Generator output should be 2x your inverter’s output
Match generator voltage to inverter voltage
Generator must be warrantied for off-grid use
2-wire start is mandatory to work automatically with solar system
3000 RPM generators are more cost-effective, while 1500 RPM generators cost more up front but last longer and are more efficient
